CN114894106A - Opaque sample thickness measurement system and method - Google Patents
Opaque sample thickness measurement system and method Download PDFInfo
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- CN114894106A CN114894106A CN202210538785.2A CN202210538785A CN114894106A CN 114894106 A CN114894106 A CN 114894106A CN 202210538785 A CN202210538785 A CN 202210538785A CN 114894106 A CN114894106 A CN 114894106A
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- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
- G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
Abstract
The invention belongs to the field of optical measurement, and relates to a system and a method for measuring the thickness of an opaque sample. The system comprises: the device comprises an SLD light source, a spectrometer, a first optical fiber coupler, a second optical fiber coupler, a third optical fiber coupler, a reference end structure and a measuring end structure. The first optical fiber coupler is respectively connected with the SLD light source and the spectrometer, and the second optical fiber coupler and the third optical fiber coupler are connected with the first optical fiber coupler; the reference end structure comprises: the device comprises an upper surface reference end probe and an adjusting frame thereof, a first reflector, a lower surface reference end probe and an adjusting frame thereof, and a second reflector; the measuring tip structure includes: the device comprises an upper surface measuring end probe and an adjusting frame thereof, a lower surface measuring end probe and an adjusting frame thereof, and a sample objective table. The invention does not need mechanical scanning, reduces the measuring time and improves the measuring efficiency. Interference spectrum signals of the upper surface and the lower surface are ensured to be in the same time, and the influence of drift of the system along with the time on a measurement result is eliminated.
Description
Technical Field
The invention belongs to the field of optical measurement, and particularly relates to a system and a method for measuring the thickness of an opaque sample based on a double-interference probe.
Background
In the field of industrial production, the thickness of the sample is a very important parameter. The thickness of the sample determines whether the sample can work normally or not to a certain extent, the mechanical property, the light transmission property, the surface structure and the like of the material are reflected by different thicknesses, and particularly, when the sample is matched with other processes, the importance is obviously exerted on the accurate control and measurement of the thickness.
Current methods for measuring the thickness of opaque samples can be classified into contact and non-contact. Probe contact measurements can cause slight damage to the surface of the sample; the thickness of a transmitted sample is measured in real time by utilizing the non-contact measurement of the X-ray transmission capacity according to the attenuation degree after the sample is transmitted, but different measured materials need to calibrate the measured value, and the X-ray has radiation to a human body and needs safety certification and maintenance cost; non-contact measurement using the eddy current effect can only achieve thickness measurement of a metal sheet, and is not suitable for a wide range of opaque samples depending on the kind of metal to be measured.
With the continuous development of the technology, the optical measurement method has started to be widely applied to the field of industrial production due to the advantages of high precision, non-contact type and the like. However, the existing optical measurement means is mostly used for measuring thin film samples, cannot measure thicker samples, and has the disadvantages of long measurement time, complex measurement system and low measurement precision.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a system and a method which can measure an opaque sample with a certain thickness, and have high measuring speed and high precision.
A first aspect of the present invention provides an opaque sample thickness measurement system comprising:
an SLD light source;
the spectrometer is used for collecting interference spectrum signals;
the first optical fiber coupler is respectively connected with the SLD light source and the spectrometer and is used for splitting the light source and collecting interference signals;
a second fiber coupler connected to the first fiber coupler;
a third fiber coupler connected to the first fiber coupler;
a column;
a reference end structure, the reference end structure comprising:
the upper surface reference end probe is connected with the second optical fiber coupler;
the upper surface reference end probe adjusting frame is used for adjusting the angle of the upper surface reference end probe and is arranged on the upright post;
the first reflector is arranged right below the upper surface reference end probe and is spaced at a certain distance;
the first reflector displacement table is used for supporting and moving the first reflector and is arranged on the upright post;
the lower surface reference end probe is connected with the third optical fiber coupler;
the lower surface reference end probe adjusting frame is used for adjusting the angle of the lower surface reference end probe and is arranged on the upright post;
the second reflector is arranged right above the lower surface reference end probe and is spaced at a certain distance;
the second reflector adjusting frame is used for adjusting the angle of the second reflector and is arranged on the upright post; a measurement end structure, the measurement end structure comprising:
the upper surface measuring end probe is opposite to the upper surface of the sample and is connected with the second optical fiber coupler;
the upper surface measuring end probe adjusting frame is used for adjusting the angle of the upper surface measuring end probe and is arranged on the upright post;
the lower surface measuring end probe is opposite to the lower surface of the sample and is connected with the third optical fiber coupler;
the lower surface measuring end probe adjusting frame is used for adjusting the angle of the lower surface measuring end probe and is arranged on the upright post;
the sample stage is used for placing a sample, is arranged between the upper surface measuring end probe and the lower surface measuring end probe, and has an XY direction displacement function;
and the displacement table is used for adjusting the XYZ directions of the lower surface measuring end probe.
Further, the first fiber coupler is a 2 × 2 single-mode fiber coupler.
Further, the second fiber coupler and the third fiber coupler are both 1 × 2 single-mode fiber couplers with a coupling ratio of 50: 50.
Furthermore, the upper surface reference end probe, the lower surface reference end probe, the upper surface measuring end probe and the lower surface measuring end probe are all single-mode GRIN fiber probes.
The second aspect of the present invention provides a measurement method based on the above measurement system, including the following steps:
1) aligning two measuring end probes which are opposite to the upper surface and the lower surface of the sample, and ensuring that measuring points of the two probes are positioned on the same vertical line of the surface of the sample;
2) using a known thickness H 0 The opaque standard sample is used for calibrating the absolute distance between the two interference systems on the upper surface and the lower surface of the measurement sample, namely the standard sample is placed, the interference spectrum signal at the moment is collected and data processing is carried out, and the absolute distance d between the reference end and the measurement end probe in the interference system for measuring the upper surface of the standard sample is obtained 1 Absolute distance d between the probe of the measuring end and the probe of the reference end in an interferometric system for measuring the lower surface of a standard sample 2 ；
3) Calculating to obtain the distance L between the zero optical path difference positions of the two interference systems as H 0 +d 2 -d 1 ；
4) After calibration is finished, a sample to be measured is placed, interference spectrum signals at the moment are collected and data processing is carried out, and the absolute distance d 'between a reference end probe and a measuring end probe in an interference system for measuring the upper surface of the sample to be measured is obtained' 1 And the absolute distance d 'between a reference end probe and a measuring end probe in the interference system for measuring the lower surface of the sample to be measured' 2 ；
5) In conclusion, the thickness H ═ L + d 'of the sample to be measured can be calculated' 1 -d′ 2 。
The step 1) comprises the following steps:
(1.1) turning on an SLD light source, putting a standard sample, adjusting a probe at the measuring end of the upper surface of the measured standard sample to be vertical to the standard sample, so that an interference system at the upper surface of the measured standard sample obtains an optimal interference spectrum signal, and adjusting a probe at the measuring end of the lower surface of the measured standard sample to be vertical to the standard sample, so that an interference system at the lower surface of the measured standard sample obtains an optimal interference spectrum signal;
(1.2) taking away the standard sample, only disconnecting the first optical fiber coupler from the interference system on the lower surface of the measurement sample, and directly connecting the probe at the measuring end on the lower surface of the measurement sample with the spectrometer, wherein the probe at the measuring end on the upper surface of the measurement sample is taken as an emergent end of a light source, and the probe at the measuring end on the lower surface of the measurement sample is taken as an incident end for receiving the light source;
(1.3) fixing the probe at the measuring end of the upper surface of the measuring standard sample, taking the probe at the measuring end of the upper surface as a reference, changing the XY directions of the displacement table, moving the probe at the measuring end of the lower surface until the value of the light power received by the probe at the measuring end of the lower surface is maximum, and judging that the two measuring probes are aligned;
and (1.4) restoring the connection of the first optical fiber coupler and the interference system on the lower surface of the measurement sample, and keeping the upper surface measurement end probe and the lower surface measurement end probe fixed in the measurement step.
The specific method for processing the interference spectrum signal to obtain the absolute distance in the steps 2) and 4) is to extract phase information of the interference spectrum signal by utilizing Fourier transform, and solve the absolute distance between a reference end and a measuring end probe by a slope method. Description of the drawings: because the spectrometer simultaneously collects the absolute distance of two interference systems on the upper surface and the lower surface of a measurement sample, before collecting interference spectrum signals, the system needs to be adjusted to ensure that d is measured 1 And d 2 And certain distance intervals are provided to avoid information overlapping. The method comprises the following specific steps:
(2.1) carrying out Fourier transform on the acquired interference spectrum signals, wherein the acquired interference spectrum signals comprise sample upper surface interference signals and sample lower surface interference signals, and transforming the signals from a time domain to a frequency domain;
(2.2) extracting amplitude information of the frequency domain signal, respectively determining signal windowing positions according to the positions of two peaks of the frequency domain signal, and extracting an effective signal containing phase information;
(2.3) performing inverse Fourier transform on the effective signal to obtain an effective time domain signal, extracting imaginary part information of the effective time domain signal to obtain a wrapped phase, and unwrapping the wrapped phase to obtain phase information phi of the interference spectrum signal;
(2.4) in the phase information Φ of the interference spectrum signal, a linear relationship between the phase and the wave number, which is introduced by the absolute distance between the measuring terminal and the reference terminal, is:
wherein d is an absolute distance,
is the wave number.
The method comprises the steps that linear fitting is carried out on phase information phi and wave number k of interference spectrum signals to obtain a slope, and the slope is divided by 4 pi to obtain a result, namely the absolute distance d between a reference end probe and a measuring end probe;
(2.5) respectively extracting effective signals according to two peaks in the frequency domain signals, then executing the steps (2.3) and (2.4), and simultaneously calculating d from the acquired interference signal spectrum 1 、d 2 A value;
and 3) in the calculation formula of the step 5), the absolute distance between the two interference systems on the upper surface and the lower surface of the sample is subtracted from one another, and actually, the measuring probes for measuring the two interference systems on the upper surface and the lower surface of the sample are adjusted to different directions by taking the zero optical path difference as a reference. This is because if the stage or the sample is tilted, the errors caused to the measuring end probes for measuring the upper and lower surfaces of the sample are the same, and the principle errors are:
in the formula, δ is an optical path from the measuring end probe to the surface of the sample, and θ is 2 times an inclination angle of the stage or the sample with respect to an alignment direction of the two measuring end probes. Therefore, the absolute distance calculated by two interference signals of the upper surface and the lower surface of the sample is subtracted, so that the error of the measurement result caused by the inclination of the object stage or the sample can be reduced.
The invention relates to a system and a method for measuring the thickness of an opaque sample by using double interference probes, which are used for obtaining the distance between zero optical path difference positions of two interference systems on the upper surface and the lower surface of a measured sample by calibrating a standard sample with known thickness by using the double probes; collecting interference spectrum information by using a spectrometer; phase information of the acquired interference spectrum signals is acquired by using a Fourier transform method, and an absolute distance is calculated and acquired by using a slope method, so that the thickness of the sample is measured. The measuring system only needs to be calibrated once, the measuring method is simple and feasible, the measuring system is small and compact in structure, and the thickness of a sample is greatly convenient to measure.
Compared with the prior art, the invention has the main advantages that:
(1) the measuring system mainly comprises optical fiber devices, so that the occupied space of the system is reduced, the influence of the external environment is reduced, the stability of the system is improved, and the system is greatly convenient to adjust.
(2) The invention does not need mechanical scanning, reduces the measuring time and improves the measuring efficiency; the invention directly processes the interference spectrum signal collected by the spectrometer, and the measurement range of the sample thickness is mainly determined by the resolution of the spectrometer, so that the use of the high-resolution spectrometer can ensure that the system has a larger measurement range.
(3) In the invention, the interference spectrum signals of the upper surface and the lower surface of the sample are simultaneously collected by the spectrometer, so that the interference spectrum signals of the upper surface and the lower surface are ensured to be in the same time, and the influence of the drift of the system along with the time on the measurement result is eliminated.
(4) The thickness calculation in the invention adopts a subtraction method, thereby effectively reducing the error of the measurement result caused by the inclination of the objective table or the inclination of the sample.
Drawings
FIG. 1 is a block diagram of a dual interference probe based opaque sample thickness measurement system of the present invention;
FIG. 2 is an optical path diagram of an opaque sample thickness measurement system based on a dual interference probe according to the present invention;
FIG. 3 is a flow chart of the present invention for obtaining absolute distance data between a reference terminal and a measuring terminal;
FIG. 4 is a schematic diagram of the present invention that accounts for stage or sample tilt errors;
FIG. 5 is an interference spectrum diagram of the spectrometer of the present invention simultaneously collecting interference signals of the upper and lower surfaces of a sample;
FIG. 6 is an explanatory diagram of absolute distances between upper and lower surfaces measured correspondingly to time-frequency domain signals for data processing according to the present invention;
in the drawings
1: an SLD light source; 2: a first fiber coupler; 3: a second fiber coupler; 4: a third fiber coupler;
5: a reference end structure; 501, a surface reference end probe; 502 upper surface reference end probe adjustment mount; 503 a first mirror; 504 a first mirror displacement stage; 505 lower surface reference end probe; 506 lower surface reference end probe adjusting frame; 507 a second reflector; 508 a second mirror adjustment mount;
6: a measurement end structure; 601 an upper surface measurement end probe; 602, a probe adjusting frame at the upper surface measuring end; 603a sample upper surface, 603b sample lower surface; 604 a sample stage; 605 lower surface measuring end probe; 606 under surface measurement end probe adjusting rack; 607 a displacement stage;
7: a spectrometer; 8: a column;
e1: reflected light of the first reflecting mirror 503 collected by the upper surface reference end probe 501;
e2: reflected light of the second mirror 507 collected by the lower surface reference end probe 505;
e3: reflected light of the sample upper surface 603a collected by the upper surface measuring end probe 601;
e4: reflected light of the sample lower surface 603b collected by the lower surface measuring end probe 605;
e5: light entering the lower surface measuring end probe 605 from the upper surface measuring end probe 601 during system calibration;
d 1: measuring the absolute distance between a reference end probe 501 and a measuring end probe 601 in the interference system on the upper surface of the standard sample;
d 2: measuring the absolute distance between a reference end probe 505 and a measuring end probe 605 in the interference system on the lower surface of the standard sample;
l: measuring the distance between the zero optical path difference positions of the two interference systems on the upper surface and the lower surface of the sample;
h: the thickness of the standard sample;
δ 1 : the upper surface measures the optical path from the end probe 601 to the upper surface of the sample;
δ′ 1 : the optical path of the upper surface sample reflected light to the upper surface measurement end probe 601;
δ 2 : the optical path from the lower surface measurement end probe 605 to the lower surface of the sample;
δ′ 2 : the optical path from the lower surface sample reflected light to the lower surface measurement end probe 605;
theta/2: the stage or sample measures the tilt angle of the end probe in the direction of alignment for the two upper and lower surface interferometric systems.
Detailed Description
It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.
It is noted that, unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
The invention provides a method for measuring the thickness of an opaque sample by using a double interference probe, belongs to an optical measurement method, and realizes point-to-point measurement of the upper surface and the lower surface of the sample. The double-interference probe measurement refers to the fact that the two pairs of interference probes are used for measuring the upper surface and the lower surface of a measured sample respectively, the two pairs of interference probes can independently measure, the spectrometer collects and records data simultaneously, the interference spectrum signals are subjected to Fourier transform, phase information is extracted, measurement of the thickness of an opaque material is further analyzed, and the double-interference probe measurement has the advantages of being non-contact, high in precision, free of mechanical scanning, fast in measurement time, large in measurement range and the like. And this system adopts the optic fibre device to build and forms, compares the discrete component, has small, receives the little advantage of external environment influence.
As shown in fig. 1, the opaque sample thickness measuring system based on the dual interference probe of the present invention comprises:
an SLD light source 1;
the spectrometer 7 is used for collecting interference spectrum signals; the output of the spectrometer 7 is connected with a computer.
The first optical fiber coupler 2 is respectively connected with the SLD light source 1 and the spectrometer 7, and is used for splitting the light source and collecting interference signals; the first optical fiber coupler 2 is a 2 × 2 single-mode optical fiber coupler;
and a second fiber coupler 3, wherein the second fiber coupler 3 is connected to the first fiber coupler 2, and the second fiber coupler 3 is a 1 × 2 single-mode fiber coupler having a 50:50 coupling ratio.
And a third fiber coupler 4, wherein the third fiber coupler 4 is connected to the first fiber coupler 2, and the third fiber coupler 4 is a 1 × 2 single-mode fiber coupler having a 50:50 coupling ratio.
And the upright column 8 is used for supporting each adjusting frame, the displacement table and the objective table.
A reference end structure 5, the reference end structure 5 comprising:
the upper surface reference end probe 501 is a single-mode GRIN fiber probe and is connected to the second fiber coupler 3;
and the upper surface reference end probe adjusting frame 502 is installed on the upright post 8 and is used for adjusting the angle of the upper surface reference end probe 501.
And the first reflecting mirror 503 is arranged right below the upper surface reference end probe 501 and is spaced at a certain distance.
And a first mirror displacement stage 504 mounted on the column 8 for adjusting the z-direction movement of the first mirror 503 to change the absolute distance between the upper surface reference end probe 501 and the upper surface measurement end probe 601.
A lower surface reference end probe 505 which is a single mode GRIN fiber probe and is connected with the third fiber coupler 4;
and the lower surface reference end probe adjusting frame 506 is installed on the upright post 8 and is used for adjusting the angle of the lower surface reference end probe 505.
And a second reflecting mirror 507 installed right above the lower surface reference end probe 505 at a certain distance.
The second reflector adjusting frame 508 is mounted on the upright post 8, mainly plays a role of supporting the second reflector 507, and can also be used for angle adjustment of the second reflector 507.
A measurement end structure 6, the measurement end structure 6 comprising:
the upper surface measuring end probe 601 is opposite to the upper surface 603a of the sample, and is connected with the second optical fiber coupler 3 by adopting a single-mode GRIN optical fiber probe;
and the upper surface measuring end probe adjusting frame 602 is installed on the upright post 8 and is used for adjusting the angle of the upper surface measuring end probe 601.
A lower surface measuring end probe 605 which is opposite to the sample lower surface 603b and is connected with the third optical fiber coupler 4 by adopting a single-mode GRIN optical fiber probe;
and the lower surface measuring end probe adjusting frame 606 is installed on the upright post 8 and is used for adjusting the angle of the lower surface measuring end probe 605.
A sample stage 604 for holding a sample. Is arranged between the upper surface measuring end probe 601 and the lower surface measuring end probe 605 and has the XY direction displacement function.
The displacement stage 607 is used to adjust the XYZ direction of the lower surface measurement end probe 605.
As shown in fig. 3 and 4, the measuring method of the opaque sample thickness measuring system based on the double interference probe of the present invention comprises the following steps:
1) aligning two upper surface measuring end probes 601 and lower surface measuring end probes 605 which are opposite to the upper surface and the lower surface of the sample, and ensuring that measuring points of the two probes are positioned on the same vertical line of the surface of the sample; the method comprises the following steps:
(1.1) turning on an SLD light source 1, putting a standard sample, adjusting a probe 601 at the measuring end of the upper surface to be perpendicular to the standard sample so that an interference system for measuring the upper surface of the standard sample can obtain an optimal interference spectrum signal, and adjusting a probe 605 at the measuring end of the lower surface to be perpendicular to the standard sample so that the interference system for measuring the lower surface of the standard sample can obtain the optimal interference spectrum signal;
(1.2) taking away the standard sample, only disconnecting the first optical fiber coupler 2 from the interference system on the lower surface of the measurement sample, and directly connecting the lower surface measurement end probe 605 with the spectrometer 7, wherein the upper surface measurement end probe 601 of the measurement sample serves as an emergent end of a light source, and the lower surface measurement end probe 605 serves as an incident end for receiving the light source;
(1.3) fixing the upper surface measuring end probe 601 of the measuring standard sample, namely, taking the upper surface measuring end probe 601 as a reference, changing the XY direction of the displacement table 607, moving the lower surface measuring end probe 605 until the value of the light power received by the lower surface measuring end probe 605 is maximum, and thus judging that the two measuring probes are aligned.
(1.4) restoring the interference system connection between the first fiber coupler 2 and the lower surface of the measurement sample, and keeping the upper surface measurement end probe 601 and the lower surface measurement end probe 605 fixed in the measurement step.
2) Using a known thickness H 0 The opaque standard sample is used for calibrating the absolute distance between the two interference systems on the upper surface and the lower surface of the measurement sample, namely the standard sample is placed, the interference spectrum signal at the moment is collected and data processing is carried out, and the absolute distance d between the probe 501 at the reference end of the upper surface and the probe 601 at the measurement end of the upper surface of the measurement sample is obtained 1 For measuring the absolute distance d between the lower surface reference end probe 505 and the lower surface measurement end probe 605 of the standard sample 2 . Since the spectrometer 7 simultaneously collects the absolute distance between the two interference systems on the upper and lower surfaces of the measurement sample, the system needs to be adjusted to allow d to be measured before collecting the interference spectrum signal 1 And d 2 And certain distance intervals are provided to avoid information overlapping. The specific data processing process is shown in fig. 3, and includes:
(2.1) carrying out Fourier transform on the acquired interference spectrum signals, wherein the acquired interference spectrum signals comprise sample upper surface interference signals and sample lower surface interference signals, and transforming the signals from a time domain to a frequency domain;
(2.2) extracting amplitude information of the frequency domain signal, respectively determining signal windowing positions according to the positions of two peaks of the frequency domain signal, and extracting an effective signal containing phase information;
(2.3) performing inverse Fourier transform on the effective signal to obtain an effective time domain signal, extracting imaginary part information of the effective time domain signal to obtain a wrapped phase, and unwrapping the wrapped phase to obtain phase information phi of the interference spectrum signal;
(2.4) in the phase information Φ of the interference spectrum signal, a linear relationship between the phase and the wave number, which is introduced by the absolute distance between the measuring terminal and the reference terminal, is:
wherein d is an absolute distance,
is the wave number.
The method comprises the steps that linear fitting is carried out on phase information phi and wave number k of interference spectrum signals to obtain a slope, and the slope is divided by 4 pi to obtain a result, namely the absolute distance d between a reference end probe and a measuring end probe;
(2.5) after respectively extracting effective signals according to two peaks in the frequency domain signals, respectively executing the steps (2.3) and (2.4), and simultaneously solving d from the acquired interference signal spectrum 1 、d 2 A value;
3) calculating to obtain the distance L between the zero optical path difference positions of the two interference systems as H 0 +d 2 -d 1 And completing calibration at this time, and only needing to calibrate once. The absolute distance between the two interference systems on the upper surface and the lower surface of the sample in the calculation formula is calculated by 'adding one and subtracting', and actually, the measuring probes for measuring the two interference systems on the upper surface and the lower surface of the sample are adjusted to different directions by taking zero optical path difference as a reference, so that the error of a measuring result caused by the inclination of an objective table or the sample can be reduced;
4) starting to measure a sample to be measured, namely after calibration is finished, putting the sample to be measured, collecting an interference spectrum signal at the moment and performing data processing to obtain an absolute distance d 'between a reference end probe 501 and a measuring end probe 601 in an interference system for measuring the upper surface 603a of the sample to be measured' 1 By usingAbsolute distance d 'between reference end probe 505 and measuring end probe 605 in interference system for measuring lower surface 603b of sample to be measured' 2 . The specific data processing process comprises the following steps:
(2.1) carrying out Fourier transform on the acquired interference spectrum signals, wherein the acquired interference spectrum signals comprise sample upper surface interference signals and sample lower surface interference signals, and transforming the signals from a time domain to a frequency domain;
(2.2) extracting amplitude information of the frequency domain signal, respectively determining signal windowing positions according to the positions of two peaks of the frequency domain signal, and extracting an effective signal containing phase information;
(2.3) performing inverse Fourier transform on the effective signal to obtain an effective time domain signal, extracting imaginary part information of the effective time domain signal to obtain a wrapped phase, and unwrapping the wrapped phase to obtain phase information phi of the interference spectrum signal;
and (2.4) dividing the slope obtained by linear fitting of the phase information phi and the wave number k of the interference spectrum signal by 4 pi, namely the absolute distance d between the reference end and the measuring end probe.
(2.5) after effective signals are respectively extracted according to two peaks in the frequency domain signals, the steps of (2.3) and (2.4) are executed, and d 'is simultaneously calculated from the acquired interference signal spectrum' 1 、d′ 2 The value is obtained.
5) In conclusion, the thickness H ═ L + d 'of the sample to be measured can be calculated' 1 -d′ 2 。
Fig. 5 shows an interference spectrum diagram of the spectrometer simultaneously collecting interference signals of the upper and lower surfaces of the sample in the embodiment.
Fig. 6 shows the absolute distance between the upper and lower surfaces measured by the data processing time-frequency domain signals in this embodiment.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (7)
1. An opaque sample thickness measurement system, comprising:
an SLD light source;
the spectrometer is used for collecting interference spectrum signals;
the first optical fiber coupler is respectively connected with the SLD light source and the spectrometer and is used for splitting the light source and collecting interference signals;
a second fiber coupler connected to the first fiber coupler;
a third fiber coupler connected to the first fiber coupler;
a column;
a reference end structure, the reference end structure comprising:
the upper surface reference end probe is connected with the second optical fiber coupler;
the upper surface reference end probe adjusting frame is used for adjusting the angle of the upper surface reference end probe and is arranged on the upright post;
the first reflector is arranged right below the upper surface reference end probe and is spaced at a certain distance;
the first reflector displacement table is used for supporting and moving the first reflector and is arranged on the upright post;
the lower surface reference end probe is connected with the third optical fiber coupler;
the lower surface reference end probe adjusting frame is used for adjusting the angle of the lower surface reference end probe and is arranged on the upright post;
the second reflector is arranged right above the lower surface reference end probe and is spaced at a certain distance;
the second reflector adjusting frame is used for adjusting the angle of the second reflector and is arranged on the upright post;
a measurement end structure, the measurement end structure comprising:
the upper surface measuring end probe is opposite to the upper surface of the sample and is connected with the second optical fiber coupler;
the upper surface measuring end probe adjusting frame is used for adjusting the angle of the upper surface measuring end probe and is arranged on the upright post;
the lower surface measuring end probe is opposite to the lower surface of the sample and is connected with the third optical fiber coupler;
the lower surface measuring end probe adjusting frame is used for adjusting the angle of the lower surface measuring end probe and is arranged on the upright post;
the sample stage is used for placing a sample, is arranged between the upper surface measuring end probe and the lower surface measuring end probe, and has an XY direction displacement function;
and the displacement table is used for adjusting the XYZ directions of the lower surface measuring end probe.
2. The opaque sample thickness measurement system of claim 1, wherein the first fiber coupler is a 2 x 2 single mode fiber coupler.
3. The opaque sample thickness measurement system of claim 1, wherein the second fiber coupler and the third fiber coupler are each a 1 x 2 single-mode fiber coupler with a 50:50 coupling ratio.
4. The opaque sample thickness measurement system of claim 1, wherein the upper surface reference end probe, the lower surface reference end probe, the upper surface measurement end probe, and the lower surface measurement end probe are single mode GRIN fiber probes.
5. A measuring method of an opaque sample thickness measuring system according to claim 1, characterized by comprising the steps of:
1) aligning the upper surface measuring end probe and the lower surface measuring end probe to ensure that the measuring points of the two probes are positioned on the same vertical line of the surface of the sample;
2) using a known thickness H 0 The opaque standard sample is used for calibrating and measuring the absolute distance between the two interference systems on the upper surface and the lower surface of the sample, namely the standard sample is put intoAcquiring interference spectrum signals at the moment and processing the data to obtain the absolute distance d between a reference end and a measuring end probe in the interference system for measuring the upper surface of the standard sample 1 Absolute distance d between the probe of the measuring end and the probe of the reference end in an interferometric system for measuring the lower surface of a standard sample 2 ；
3) Calculating to obtain the distance L between the zero optical path difference positions of the two interference systems as H 0 +d 2 -d 1 ；
4) After calibration is finished, a sample to be measured is placed, interference spectrum signals at the moment are collected and data processing is carried out, and the absolute distance d 'between a reference end probe and a measuring end probe in an interference system for measuring the upper surface of the sample to be measured is obtained' 1 And the absolute distance d 'between a reference end probe and a measuring end probe in the interference system for measuring the lower surface of the sample to be measured' 2 ；
5) Calculating to obtain the thickness H ═ L + d 'of the sample to be measured' 1 -d′ 2 。
6. The method according to claim 5, characterized in that step 1) comprises in particular the steps of:
(1.1) turning on an SLD light source, putting a standard sample, adjusting a probe at the measuring end of the upper surface of the measuring standard sample to be vertical to the standard sample so as to enable an interference system at the upper surface of the measuring standard sample to obtain an optimal interference spectrum signal, and adjusting a probe at the measuring end of the lower surface of the measuring standard sample to be vertical to the standard sample so as to enable an interference system at the lower surface of the measuring standard sample to obtain an optimal interference spectrum signal;
(1.2) taking away the standard sample, only disconnecting the first optical fiber coupler from the interference system on the lower surface of the measurement sample, and directly connecting the probe at the measuring end on the lower surface of the measurement sample with the spectrometer, wherein the probe at the measuring end on the upper surface of the measurement sample is taken as an emergent end of a light source, and the probe at the measuring end on the lower surface of the measurement sample is taken as an incident end for receiving the light source;
(1.3) fixing the probe at the upper surface measuring end of the measuring standard sample to be fixed, and moving the probe at the lower surface measuring end in the XY direction of the displacement table by taking the probe at the upper surface measuring end as a reference until the value of the light power received by the probe at the lower surface measuring end is maximum, so as to judge that the two measuring probes are aligned;
and (1.4) restoring the connection of the first optical fiber coupler and the interference system on the lower surface of the measurement sample, and keeping the upper surface measurement end probe and the lower surface measurement end probe fixed in the measurement step.
7. The method according to claim 5, wherein the specific method for processing the interference spectrum signal to obtain the absolute distance in the steps 2) and 4) is to extract phase information of the interference spectrum signal by using Fourier transform, and the absolute distance between the reference end and the measuring end probe is calculated by a slope method, and the specific steps include:
(2.1) carrying out Fourier transform on the acquired interference spectrum signals, wherein the acquired interference spectrum signals comprise sample upper surface interference signals and sample lower surface interference signals, and transforming the signals from a time domain to a frequency domain;
(2.2) extracting amplitude information of the frequency domain signal, respectively determining signal windowing positions according to the positions of two peaks of the frequency domain signal, and extracting an effective signal containing phase information;
(2.3) performing inverse Fourier transform on the effective signal to obtain an effective time domain signal, extracting imaginary part information of the effective time domain signal to obtain a wrapped phase, and unwrapping the wrapped phase to obtain phase information phi of the interference spectrum signal;
(2.4) in the phase information Φ of the interference spectrum signal, a linear relationship between the phase and the wave number, which is introduced by the absolute distance between the measuring end and the reference end, is as follows:
wherein d is an absolute distance,
is the wave number;
the method comprises the steps that linear fitting is carried out on phase information phi and wave number k of interference spectrum signals to obtain a slope, and the slope is divided by 4 pi to obtain a result, namely the absolute distance d between a reference end probe and a measuring end probe;
(2.5) respectively extracting effective signals according to two peaks in the frequency domain signals, then executing the steps (2.3) and (2.4), and simultaneously calculating d from the acquired interference signal spectrum 1 、d 2 The value is obtained.
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